Accretion-Induced Conversion of High-Velocity Neutron Stars to Strange Stars in Supernovae and Implications for Gamma-Ray Bursts

We present a new model for gamma-ray bursts (GRBs) that are not only associated with supernovae but also have small baryon contamination. In this model, we assume a newborn neutron star to move outward at a kick velocity of $\sim 10^3 {\rm km} {\rm s}^{-1}$ in the supernova ejecta. We find that such a neutron star still hypercritically accretes its surrounding supernova matter. Once the stellar mass increases to some critical mass, the neutron star will undergo a phase transition to become a strange star, leading to an energy release of a few $10^{52}$ ergs. The phase transition, if possibly occuring just near the supernova front, will first result in an ultra-relativistic fireball and then a GRB. This provides a plausible explanation for the GRB-supernova association. We estimate the burst rate to be $\sim 10^{-6}$ per year per galaxy. Our model also predicts other possiblities. For example, if the resulting fireballs have a Lorentz factor of the order of a few, they will produce X-ray GRBs observed by BeppoSAX. We find the rate of such bursts to be $\sim 10^{-5}$ per year per galaxy.